1. Field of the Invention
The present invention relates to a focusing tool and in particular to provide a fast, sensitive, and robust technique of performing autofocus in a tool.
2. Related Art
Current technologies for obtaining a good focus in tools include, for example, beam displacement or confocal depth sensing. The beam displacement technique typically used for the mask or wafer (generically referenced herein as a sample) has the undesired property of being confused by the presence of dense patterns on the sample. The confocal technique helps to overcome this problem because of its smaller inherent spot and better focus resolution. In a typical system, a laser generates a collimated beam that is deflected by beam splitter and then focused by an objective lens onto the sample as a single point. In a confocal system, the reflected light from the sample is recollimated into a confocal beam, which is then focused by another lens through a pin hole. Assuming the system is focused, the optical image at the pin hole is identical to that on the sample.
A detector can use this optical image from the confocal beam to generate a V(z) curve of that particular position on the sample, wherein a V(z) graph plots voltage as a function of height z. Note that in a V(z) graph, a maximum voltage corresponds to a maximum amount of light when a spot is focused on the sample. Therefore, by monitoring the maximum voltage, a positional focus can be obtained.
Depending on the type of response obtained, the confocal system either “accepts” the spot on the sample as “valid”, or “rejects” the spot and moves on to another location. Note that the term “valid” means that the experimentally obtained V(z) curve is similar to that for a single, edgeless surface. FIG. 1A illustrates an exemplary V(z) curve 101 assuming a single, edgeless surface. Note that each material forming part of the surface of the sample has a unique intensity for its V(z) curve.
When focusing in a dense pattern area, a single beam/spot has a severe limitation in that the V(z) curve is corrupted by edges in the dense pattern. Specifically, the signal from the single beam scatters because of edges or corners in the dense geometry, thereby drastically reducing the quality of the signal. Moreover, a dense geometry will result in interference within a single point, which gives rise to certain fluctuations in the V(z) curve. Thus, any inference drawn about the location of the focal plane using a single beam/spot, particularly when a dense pattern area is being interrogated, is suspect.
For example, if the spot happens to fall on an edge or a corner of a feature or on a defect on the sample, then the response is mildly or even grossly different from an idealized V(z) curve (for the given numerical aperture (NA) of the lens and wavelength). For example, FIG. 1B illustrates a spot 102 that is positioned at an edge 103 of a feature on a sample. FIG. 1C illustrates an exemplary V(z) curve 104 assuming the edge condition shown in FIG. 1B.
This response results in ambiguous information about the exact location of the focal plane. For example, V(z) curve 104 indicates that spot 102 is not focused on either of the two surfaces shown in FIG. 1B. Thus, V(z) curve 104 can be characterized as “non-conformal” to V(z) curve 101.
When the response appears to have been due to the spot position being in a non-ideal location, the general practice is to move to an adjacent location and try again. Because there is no guarantee that the next location will be in a relatively uniform area of the pattern, this repetition is highly inefficient. Logically, this problem becomes more severe for more densely patterned areas of the mask. Indeed, if the level of pattern density exceeds a predetermined amount, it may be impossible to have any reasonable V(z) response in a given location.
In addition, note that the accuracy with which the focal plane can be determined is governed by the available signal to noise ratio, which can vary considerably for different locations. Note that although in theory a signal to noise ratio using a single beam can be improved by simply increasing the laser power, this approach is severely limited in practice. Specifically, increasing the laser power can result in significant sample damage. Moreover, the detector in this system may quickly reach its saturation level using this approach.
Thus, defining what is meant by “focus”, even under normal situations and using prevalent techniques, can be a very complicated issue. That is, the definition of focus as a place where the best signal or sharpest response is obtained is predicated on a number of assumptions, any of which can be violated.
Therefore, a need arises for a technique that can determine the focal plane without searching adjacent locations, provide for an enhanced signal to noise ratio, and establish a robust experimental definition of “best focus” regardless of the nature of the sample surface.